With the socioeconomic and urban development of Chongming Island (the largest alluvial island in the world), water demand is rapidly growing. To make adjustments to the water utilization structure of each industry, allocate limited water resources, and increase local water use efficiency, this study performed a scenario analysis for the water sustainability of Chongming Island. Four different scenarios were performed to assess the water resource availability by 2020. The growth rate for water demand will be much higher than that of water supply under a serious situation prediction. The water supply growth volume will be 2.22×108m3 from 2010 to 2020 under Scenario I and Scenario II while the corresponding water demand growth volume will be 2.74×108m3 and 2.64×108m3, respectively. There will be a rapid growth in water use benefit under both high and low development modes. The water use benefit will be about 50CNY/m3 under Scenarios I and II in 2020. The production structure will need to be adjusted for sustainable utilization of water resources. Sewage drainage but not the forest and grass coverage rate will be a major obstacle to future development and environmental quality. According to a multi-level fuzzy comprehensive evaluation, Scenario II is finally deemed to be the most desirable plan, suggesting that the policy of rapid socioeconomic development and better environmental protection may achieve the most sustainable development of Chongming Island in the future.